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Coherent long-range laser radar (LADAR) systems rely on externally supplied data to cue the RF receiver processor to examine received data at a known time and frequency. Such externally supplied data can include target angle sensor data, target range, target range rate, and target velocity. This reliance on externally supplied target data reduces the utility of this type of long range LADAR as a stand-alone autonomous system. The externally supplied data is needed in these types of systems to limit the volume of space to be searched for long-range fast-moving targets to keep the amount of data to be stored and processed to a manageable level.
In addition, LADAR systems having an all range capability would require multiple pulses to overcome speckle using pulse tone acquisition waveforms. Speckle occurs due to the surface roughness of the target having a greater dimension than the wavelength of the laser radar used to detect the target. This leads to interference in which a speckle null may occur and a target missed or lost until the target orientation relative to the the LADAR receiver has changed sufficiently to shift the interference from the null. Speckle is typically overcome by the use of multiple pulses of laser radar energy to ensure that the probability of detection of a target is nearly 100%.
Range to a target can be found by removing the Doppler shift of the return pulse and correlating the return pulse with the transmitted pulse to determine the phase shift of the return pulse. This allows the time the pulse took to travel to and from the target to be accurately determined and therefore the range of the target may be accurately estimated. This process requires the identification and removal of the Doppler shift from the return pulse.
Typically to remove the Doppler shift from a signal, the spectrum of the signal is electrically analyzed. One method of analyzing the spectrum of a signal is to measure the energy of the signal of interest in each of a plurality of narrow frequency bands. The sensitivity and accuracy of this method is dependent upon the number and the width of the selected frequency bands of a corresponding plurality of band-pass filters. This process typically uses a superheterodyne receiver and is referred to as a swept spectrum analysis. However, swept spectrum analysis does not monitor or measure all frequencies at all times. As the sensitivity of the spectrum analyzer is increases by increasing the number of bandpass filters and reducing the bandwidth of each of the filters, the time required to sweep through all of the resulting frequency bands is increased. Accordingly, the probability of intercepting a given signal is less than 100%. In an environment in which frequency-hopping systems are used, some signals are likely to be missed at least some of the time.
Another method of spectrum analysis also uses narrow bandpass filters, but in this method, a non-linear device is coupled to the output of the corresponding bandpass filter. The non-linear device provides an output that is dependent on the energy contained within the passband of the corresponding filter channel. The outputs of each of the non-linear devices are integrated over a time interval consistent with the passband width of the bandpass filter, and the outputs are multiplexed and sampled at rate to ensure a probability of intercept near 100%. However, since the frequencies of interest are typically in the radio frequency (RF) region the component values for each of the required bandpass filters can be awkward. In addition to ensure that there is no corruption of the filtered signals, each bandpass filter must have sufficient stop-band attenuation to prevent crosstalk signal interference from adjacent bands. Accordingly, the bandpass filters must be of a sufficiently high order to provide the necessary stop-band attenuation to suppress these adjacent signals. The number of components may also increase with the order of the filters so that these filters increase in size and the rate of power consumption increases with the order of the filters.
The above prior art methods are primarily analog in nature, so that the output of the various systems is nearly instantaneous. However in some applications, analog systems are inherently less accurate than digital systems although analog systems can provide a speed advantage over digital systems. Digital systems have been employed in these systems requiring high accuracy in order to provide the required highly accurate data. In particular, Fast Fourier transforms (FFTs) can be used to determine the spectral energy within one or more channels of interest. However, searching a three-dimensional volume of space for targets and attempting to detect and estimate the range and Doppler shift of a target requires the storage and processing in real-time of an extremely large amount of data.
Therefore, what is needed is a system that allows for the detection, estimation of the range, and estimation of the Doppler shift of a target that is simple and does not require complex analog filters or the storage and real-time processing of large amounts of data.
An optical correlator is used to detect and discriminate the presence of a target in a laser detection and ranging (LADAR) system by analyzing the return signal, and to provide initial estimates of the targets range and velocity to the LADAR receiver. The optical correlator includes an acoustic optical Bragg cell that diffracts/deflects an input laser beams using the received and down-converted LADAR signal. A set of integrating photodetectors is disposed to receive the diffracted/deflected laser beam output of the acoustic optical Bragg cell. The set of integrating photodetectors integrates the diffracted/deflected laser beam output of the Bragg cell over time and the integrated output is sampled at a predetermined sampling period to uncorrelate the target from noise. An optical processor analyzes and processes the data output from the set of integrating photodetectors. If one of the integrated outputs exceeds a predetermined threshold, a valid target has been detected. The physical location of the particular integrating photodetector that has received the diffracted/deflected laser beam output of the Bragg cell is indicative of the Doppler shift and hence the velocity of the detected target. The time of detection of the target is indicative of the range to the target. This allows the range-Doppler-amplitude of the target to be estimated and provided to the receiver to allow for more accurate processing of the receiver data. In addition, the optical correlator can be used to provide whole body Doppler and range estimates of the target and can also inherently average the speckle data.
In one embodiment, a hybrid processor for detecting a target at an intermediate frequency (IF) signal includes a correlator RF drive module that receives the IF signal and is configured and arranged to up-convert the IF signal into a correlator drive signal. The correlator drive signal has a correlator frequency and further is a band limited signal. The correlator drive signal is coupled to an optical spectrum analyzer/correlator.
The optical spectrum analyzer/correlator includes a laser source providing a laser beam, an acousto-optical Bragg cell having an input receiving, the correlator drive signal, and collimating optics disposed between the laser source and the acousto optical Bragg cell. The collimating optics are configured and arranged to collimate the laser beam and to provide the collimated laser beam to be incident on the acousto-optical Bragg cell. The acousto-optical Bragg cell is responsive to the correlator drive signal by diffracting/deflecting the incident collimated laser beam, and wherein the acousto-optical Bragg cell to provides a plurality of diffracted/deflected output laser beams and an undiffracted/undeflected output laser beam. An optical system is configured and arranged to receive the diffracted/deflected laser beam and to perform a Fourier transform on the diffracted/deflected laser beam and to image the plurality of the diffracted Fourier transformed laser beams onto an image plane. An optical integrating photodetector array that includes a plurality of integrating photodetectors that are disposed within the image plane provide a plurality of output signals, each signal corresponding to one of the plurality of integrating photodetectors. An optical processor is coupled to the optical photodetector array and receives the plurality of output signals therefrom, and is operative to process the plurality of output signals to detect a target and to provide an output cueing signal indicative of a detected target. In addition, the optical processor can analyze the received data to estimate the Doppler shift of the target and the range of the target.
Other forms, features and aspects of the above-described methods and system are described in the detailed description that follows.